1. Field of the Invention
The present invention relates to a method of separating and recovering Americium (Am), Curium (Cm), Zirconium (Zr), Molybdenum (Mo), Palladium (Pd) and rare earth elements from radioactive liquid wastes.
2. Description of the Related Art
Generally, elements such as Americium (Am), Curium (Cm), Zirconium (Zr), Molybdenum (Mo), Palladium (Pd) and rare earth elements are included in liquid wastes derived from high-level radioactive wastes which are produced from spent nuclear fuel reprocessing and from nuclear substance producing and dissolving steps in nuclear power facilities, for example.
The high-level radioactive wastes produced in the nuclear power facilities are defined as radioactive wastes containing primarily the products of nuclear fission and transuranic elements such as radioactive elements having atomic number higher than 92. The product of nuclear fission and the transuranic elements are produced when valuable Uranium (U) or Plutonium (Pu) is recovered from the reprocessed spent nuclear fuel from nuclear power plants. The high-level radioactive wastes are produced in a liquid state from such reprocessing. In a spent nuclear fuel reprocessing method which is designated as a Pulex method performed on an industrial scale at present, a solvent-extraction method is used to recover Uranium and Plutonium which is extracted and seperated. The method includes a first step of dissolving the spent nuclear fuel in nitric acid and a later step of extracting Uranium and Plutionium using tri-butyl phosphate (hereafter, referred as TBP) as an extraction agent. The various kinds of products of nuclear fission and the transuranic elements contained in a nuclear fuel solution are present in the remainder of the extraction process, and this remainder is produced as a high-level radioactive waste. In a step of dissolving the spent nuclear fuel or a step of treating a remainder of the nuclear fuel solution, the high-level radioactive waste as described above is also produced. Furthermore, in some overseas institutions, high-level radioactive waste as described above is also produced when a nuclear substance such as Uranium or Plutonium is produced or dissolved.
Moreover, in regard to the high-level radioactive waste as described above, a disposition plan is underway. The plan includes a step of recovering the radioactive liquid waste using nitric acid, a step of concentrating the liquid waste by vaporization, a step of working the liquid waste into a glass solidification body, and a final step of storing the glass solidification body in a deep stratum.
The high-level radioactive liquid waste coexists with about forty nuclides in addition to a small amount of Uranium and Plutonium which is not able to be recovered completely using the reprocessing as described above. The nuclides include alkali metals such as Cesium (Cs), alkaline-earth metals such as Strontium (Sr), Barium (Ba), rare earth elements such as Neodymium (Nd), Cerium (Ce), Promethium (Pm), Yttrium (Y), minor actinide elements such as Neptunium (Np), Americium (Am), Curium (Cm), platinum-group metals such as Palladium (Pm), Rhodium (Ph), Ruthenium (Ru), Zirconium (Zr), Molybdenum (Mo), Niobium (Nb), Technetium (Tc). It is very important to separate various elements contained in the high-level radioactive liquid wastes into some element groups according to levels of radioactivity, the life span or exothermic properties of the elements in order to formulate a rational method of processing these substances with reference to the following parameters. These parameters are cost effectiveness and efficiency improvements in the disposition of wastes, the reduction of environmental loads, and the effective use of resources, for example.
Especially, it is an urgent necessity that technology with respect to separation and recovery from high-level radioactive liquid wastes of minor actinide elements such as Americium, Curium which are long-life nuclear species having a half-life of more than ten thousand years is established. It is useful that the established technology contributes to reduce the radioactive load on the environment in the long term and to improve the cost effectiveness and efficiency improvements in disposition of radioactive wastes in the deep stratum.
In certain countries worldwide, the development of the so-called separation-transformation technology has proceeded vigorously in recent years. This technology includes a step of separating and recovering the minor actinides such as Americium, Curium or the like from the high-level radioactive liquid wastes and a step of transforming the minor actinides to stable or short-life nuclear species in a nuclear reactor or accelerator. However, a method of effectively separating and recovering the minor actinides such as Americium, Curium or the like from the high-level radioactive liquid wastes is not yet established under present circumstances. Especially, since Americium or Curium and the rare earth elements in the high-level radioactive liquid wastes show similarity with respect to atomic structure and chemical properties, it is difficult to separate one from the other. Up until this time, research and development regarding various separation methods, which includes the solvent-extraction method using the extraction agent, have been vigorously pursued with the goal of separating and recovering Americium and Curium from the high-level radioactive liquid wastes. However, a separating and recovering method which satisfies cost effectiveness and efficiency has not yet been developed, and is not yet in actual use on an industrial scale under present circumstances.
A typical solvent-extraction method is the well-known TRUEX method. The TRUEX method includes a step of dissolving octyl(phenyl)-N,N-diisobutylcarbamoylmethylphosphine oxide (hereafter, referred as CMPO) and TBP mixed solvent in hydrocarbon base solvent such as dodecane or the like to prepare an organic solvent, and a step of bringing the high-level radioactive liquid wastes into contact with the organic solvent (hereafter, referred as CMPO-TBP mixed solvent or organic phase) to extract and separate the Americium and Curium from the liquid wastes. In other words, according to the TRUEX method, three valences of actinides such as Americium and Curium are extracted in the CMPO-TBP mixed solvent. On the other hand, the great majority of metal elements in the high-level radioactive liquid wastes is not extracted in the CMPO-TBP mixed solvent, and remains in an aqueous phase. The extraction property of three valences of rare earth elements in the CMPO-TBP mixed solvent is however similar to that of the three valences of actinides. Since the rare earth elements are therefore extracted together with the Americium and the Curium, it is impossible to separate the three valences of actinides from the rare earth elements.
In order to separate the three valences of actinides from the rare earth elements in the high-level radioactive liquid wastes in recent years, a new solvent-extraction method is proposed (referred to JP-A-80194/9). The new method uses the CMPO-TBP mixed solvent in combination with aminopolyacetic acid based complex such as diethylenetriaminepentaacetic acid (hereafter, referred as DTPA). With the method, initially the high-level radioactive liquid wastes are mixed with the CMPO-TBP mixed solvent, and both the three valences of actinides and the rare earth element in the liquid wastes are extracted in the CMPO-TBP mixed solvent. Salting-out agents such as sodium nitrate (NaNO3) are then added to the organic phase to cleanse and remove the nitric acid from the organic phase. A solution, which contains the three valences of actinides, DTPA having complex-forming ability enhanced and the salting-out agent, is then added to the organic phase to extract backward the three valences of actinides from the organic phase to the aqueous phase. In this way, since the rare earth elements remain in the organic phase, the three valences of actinides are separated from the rare earth elements. According to the method, it is possible to separate the three valences of actinides and the rare earth elements from the high-level radioactive liquid wastes, and to separate nicely the three valences of actinides from the great majority of the rare earth elements (light rare earth elements as main elements).
Since the salting-out agent including several mole per liter of sodium nitrate is however used in the step of separating the three valences of actinides from the rare earth elements, the method has a problem producing a radioactive waste containing high concentration of salts difficult to dispose. Since a high concentration of TBP of more than 1 mole per liter is dissolved in a organic diluent such as dodecane in addition to the CMPO as the organic solvent, the method entails the problem that it produces a large amount of the organic liquid waste which poses processing difficulties in later steps. Furthermore, with the method of using the CMPO-TBP mixed solvent, Zirconium, Molybdenum and Palladium contained in the high-level radioactive liquid wastes are extracted together with the three valences of actinides and the rare earth elements in the organic solvent. It is therefore impossible to separate Zirconium, Molybdenum and Palladium from the three valences of actinides and the rare earth elements.
Moreover, a typical method of separation Americium and Curium from the rare earth elements includes a separation method using extraction chromatography instead of the solvent-extraction method described above. The extraction chromatography is performed between a solid phase, which impregnates a porous carrier with the CMPO and the TBP, and an aqueous phase, which includes an organic acid-based complex agent such as DTPA (referred to JP-A-113689/9). With the separation method, a disposal target is a mixed product solution containing Americium, Curium and the rare earth elements, which are separated from other elements in the high-level radioactive liquid wastes according to the well-known TRUEX method. The mixed product solution is passed through the solid phase taking the form of a column so that the Americium, the Curium and the rare earth elements are absorbed in the solid phase. The method separates Americium and Curium from the tare earth elements, with the help of variations in moving speed of each individual element ion in the column when the solution passes through the solid phase. With the method, when the complex agent solution such as DTPA is passed through the solid phase, Americium, Curium and a small amount of the rare earth elements (mainly heavy rare earth elements) are removed from the column in a former stage. On the other hand, a large amount of the rare earth elements (mainly light rare earth elements) is removed from the column in a later stage, and it is possible to separate the former from the later.
Separation of the three valences of actinides from the rare earth elements according to the method above is based on the difference in the complex-forming ability between Americium, Curium and the rare earth elements and the complex agent-such as DTPA. It is therefore possible to perform a neat separation of the three valences of actinides from the large amount of the rare earth element (primarily light rare earth elements). However, if the method above is applied directly to the separation Americium and Curium from the rare earth elements in high-level radioactive liquid wastes, absorptive fission products, which include Zirconium, Molybdenum or Palladium, are also absorbed in the CMPO absorbent. Furthermore, the fission products are separated together with Americium and Curium from the absorbent owing to the DTPA. It is therefore impossible to separate Americium and Curium from the absorptive elements. In order to apply the method above to the separation Americium and Curium from the rare earth elements, it is necessary to perform a previous process of separating and recovering Americium, Curium and the rare earth elements from the high-level radioactive liquid wastes. If such a previous solvent-extraction process is performed, the cost effectiveness of and the improvements to the efficiency of the whole separating-recovering process undergo remarkable deterioration as additional separation facilities to the existing ones must be added and the volume of process solution, and the varieties and amount of separation reagent in the new process are increased noticeably. The new process furthermore results in increasing markedly the volume of the radioactive liquid wastes difficult to dispose in a later step.
Moreover, a proposed method includes a step of absorbing the transuranic elements such as Americium and Curium from the radioactive liquid wastes in an absorbent and a step of separating the elements from the absorbent using the dilute nitric acid to recover them (referred to JP-A2-97155/7 and JP-A2-97156/7). Here, the absorbent is prepared by impregnating a resinous particle supporting body such as Amber-Light XAD-4 with the CMPO, dihexyl-N,N-diethylcarbamoylmethphosphonate (hereafter, referred as CMP) or a mixture of the substances with TBP. According to the method, it is possible to separate the absorptive elements such as Americium and Curium from non-absorptive elements such as Strontium or iron in the high-level radioactive liquid wastes. With the method, since the rare earth elements have approximately the same property of absorption as Americium or Curium with respect to the CPM or the CMPO and furthermore have approximately the same property of separation as Americium or Curium when the dilute nitric acid is used. Therefore, it is impossible to separate the rare earth elements from Americium or Curium. Moreover, it is also impossible to separate and recover the absorptive elements such as Zirconium, Molybdenum or Palladium, which are absorbed in the CMP or the CMPO.
Moreover, oxides of Zirconium or Molybdenum have high melting points, respectively. When the glass solidification body is made from the high-level radioactive liquid wastes, it is necessary to raise a heat temperature according to the melting point of the oxide above. Furthermore, since the oxides of Zirconium or Molybdenum weaken the mechanical strength of the glass solidification body, content of the wastes in the glass must be limited. Therefore, it comes to increase in volume of the glass solidification body as a whole.
On the other hand, Zirconium and Molybdenum contained in the high-level radioactive liquid wastes are almost stable nuclear species having low levels of radioactivity. If Zirconium and Molybdenum are separated from the high-level radioactive liquid wastes, the separated product may be disposed acceptably as low-level radioactive liquid wastes. However, since Zirconium and Molybdenum contained in the high-level radioactive liquid wastes have complex chemical configurations or complex behavior, effective separation and elimination methods have not been developed at the present time.
Furthermore, Palladium contained in the high-level radioactive liquid wastes is deposited as metals under high temperature by an application of heat. The deposited Palladium displays a conspicuous tendency to prevent the preparation of a homogeneous solidification body.
On the other hand, the Palladium contained in the high-level radioactive liquid wastes is of interest due to the fact that it is a potential resource with extremely high levels of radioactivity and having great value when used in electrode materials such as fuel cells and in chemical catalysis. In order to separate and recover the Palladium, research and development with respect to electrolytic reduction methods, solvent-extraction methods or ion-exchanging methods are under continual development. However, separation and elimination methods, which are adequate with respect to cost effectiveness and efficiency, have not yet been developed.
As described above, with the conventional method of separating and recovering Americium and Curium from the high-level radioactive liquid wastes produced from spent nuclear fuel reprocessing and from nuclear substance producing and dissolving steps in nuclear power facilities, the specific elements cannot be separated economically from another with efficiency. Consequently there is a problem that the conventional method has a possibility of preventing the development of the separation-transformation technology.
Moreover, with the conventional method, if Zirconium, Molybdenum and Palladium are contained in the high-level radioactive liquid wastes, there is a possibility that these elements will prevent the process of preparing the glass solidification bodies and furthermore result in increases in the used amount of the glass solidification material. This point constitutes one of the principal factors preventing cost effectiveness and efficiency improvements in processing of high-level radioactive liquid wastes.
With the conventional method, Palladium cannot be separated and recovered from the high-level radioactive liquid wastes. The conventional method has a further problem that the valuable Palladium resource is not used effectively.